Large-scale electrical energy storage systems are needed to accommodate the intrinsic variability of energy supply from solar and wind resources. Such energy storage systems will store the excess energy during periods of electricity production, and release the energy during periods of electricity demand. Viable energy storage systems will have to meet the following requirements: (i) low installed-cost of <$100/kWh, (ii) long operating life of over 5000 cycles, (iii) high round-trip energy efficiency of over 80%, and (iv) ease of scalability to megawatt-hour level systems. Rechargeable batteries are particularly suitable for such large-scale storage of electrical energy because of their high round-trip efficiency and scalability. Among the types of rechargeable batteries under consideration are vanadium-redox, sodium-sulfur, zinc-bromine, zinc-air and lithium-ion batteries. In addressing the challenges of durability, cost, and large-scale implementation of the foregoing types of batteries, the beneficial features of iron-based alkaline batteries for large-scale energy storage have been largely overlooked.
Nickel-iron batteries have been used in various stationary and mobile applications for over 70 years in the USA and Europe until the 1980s when the iron-based batteries were largely supplanted by sealed lead-acid batteries. Because of their high specific energy, iron-air batteries underwent active development for electric vehicles and military applications in the 1970s after the “oil shock” but major research in this area was abruptly discontinued after 1984 and with few exceptions iron electrodes have not received significant attention since that time. However, despite being less conspicuous in common applications, iron-based alkaline batteries such as iron-air and nickel-iron batteries have unique characteristics that make them very attractive and highly suitable for meeting the emerging need of grid-scale electrical energy storage systems.
The electrochemistry of the iron electrode in alkaline batteries involves the redox process involving iron (II) hydroxide and elemental iron:Fe(OH)2+2e−⇄Fe+2OH−Eº=−0.877V  (1)
The forward reaction occurs during charging of the electrode and the reverse reaction occurs during discharge.
Iron, the primary raw material for iron-based battery systems, is globally abundant, relatively inexpensive, easily-recycled, and eco-friendly. Also, the iron electrode is well-known for being robust over repeated cycles of charge and discharge. Stable performance over 3000 charge and discharge cycles has been demonstrated in nickel-iron batteries. Such robustness is extraordinary as most rechargeable battery electrodes degrade within 1000 cycles. The robustness of the iron electrode is attributed to the low solubility of the hydroxides of iron in alkaline media. The principal limitation of the iron electrode is its low charging-efficiency that is in the range of 55-70%. This limitation arises from the wasteful hydrogen evolution that occurs during charging according to the following reaction:2H2O+2e−⇄H2+2OH−Eº=−0.828V  (2)
The hydrogen evolution reaction occurs because the electrode potential for this reaction is positive to that of the iron electrode reaction (Eq. 1). Consequently, batteries will have to be overcharged by 60-100% to achieve full capacity. The hydrogen evolution that occurs during charging is undesirable because it lowers the round-trip energy efficiency and results in loss of water from the electrolyte. Thus, suppressing hydrogen evolution at the iron electrode has far-reaching benefits of raising the overall energy efficiency, lowering the cost, and increasing the ease of implementation of iron-based batteries in large-scale energy storage systems. However, suppressing hydrogen evolution and achieving an iron electrode with a charging-efficiency close to 100%, without interfering with the other performance features of the electrode, has been a formidable challenge for many years.
Another limitation of commercially available iron batteries is their inability to be discharged at high rates; when discharged in less than five hours (also termed the five-hour rate), the capacity realized is very small. Grid-scale electrical energy storage requires that the battery be capable of being charged and discharged in one to two hours. The discharge rate capability of the iron electrode can be improved if the passivation by the electrically non-conductive iron (II) hydroxide, (the discharge product) can be mitigated (Eq. 1). Shukla et al have demonstrated the beneficial role of various additives on mitigating passivation (K Vijayamohanan et al., J. Electroanal. Chem., 289, 55 (1990); T S Balasubramanian, J. Appl. Electrochem., 23, 947 (1993)). However, achieving high rate capability and high efficiency simultaneously continues to be a challenge.